1. Technical Field of the Invention
This inventions relates to a cooling and counterpressure or backpressure procedure for manufacturing plastic parts by the injection process, in which it is important to obtain a good appearance one of the sides of the part, the visible side, which might or might not be occupied by a decorative coating, which could be a plastic film, plastic foam, textile fabric, etc.
2. Prior Art
One of the most commonly used processes for obtaining plastic parts of this type, whether coated or not, is conventional plastic injection and, within this general technique, the variants that make it possible for the injection pressure, sequential injection and injection-compression processes to be lowered.
These processes consist of the following phases:
Placing the coating inside the mould, when dealing with a coated part.
Total or partial closure of the mould (the latter in the injection-compression process).
Injection of the material into the cavity of the mould.
Total closure of the mould (only in the case of injection-compression).
Compacting.
Cooling, based on the circulation of fluids inside the cooling circuits in the mould or other cooling systems.
Opening of the mould.
Ejection or removal of the part.
As indicated previously, in some processes, and especially for parts for the automotive industry, a coating is applied inside the cavity in order to decorate and/or insulate the component.
During the mould filling phase, if a coating is applied inside the cavity, the appearance of this coating deteriorates due to the high temperature of the material when it is injected and the high pressures which are created inside the cavity.
When the part is not coated and in order to perfectly copy the half of the mould that corresponds to the visible side of the part and also to compensate for the contracting of the material, it becomes necessary, once the filling of the cavity in the mould has finished, to apply a pressure called compacting or compaction pressure.
After this phase, the mould is cooled for a certain time, in a process in which this cooling time directly affects the productivity of the mould and, as a result, the final cost of the part to be produced.
Within the field of plastic injection moulding, a method is known though U.S. Pat. No. 3,345,687 in which air at low pressure is injected into the cavity in the mould through a composite material, simultaneously with the injection of the plastic material into the mould cavity or immediately after the injection while the thermoplastic material is hot.
With the application of this technique, an appropriate counterpressure is not achieved on the parts being produced, so that, in the case of coated parts, the coating is not suitably protected, whereas in the case of non-coated parts, unwanted sinkholes and warpage are not avoided.
Moreover, it has been found to be insufficient in practice, due to the relative slowness of the cooling process, which results in a cost of a certain consideration for each part produced.
In the same way, with this technique, a good surface quality in the finish of the parts is not achieved, whether coated or non-coated, and it has been found that in the case of coated parts, the coating has been substantially altered in relation to its initial appearance, which is the required finish.